Metrics  
To measure captioning performance, we adopt the captioning evaluation toolkit [1]} to calculate the standard metrics: SPICE [2]}, CIDEr [3]}, ROUGE [4]}, METEOR [5]} and BLEU [6]}, among them,
SPICE, which is based on scene graph matching, and CIDEr, which is built upon on n-gram matching, are specifically designed to evaluate captioning systems and are more likely to be consistent with human judgment [3]}, [2]}.
To measure the grounding performance, we adopt the metrics \(\text{F1}_\text{all}\)  and \(\text{F1}_\text{loc}\)  [9]} which evaluates based on two factors: whether the correct word is generated and whether the correct image region is grounded.
